1284 



BUIvIvETIN OF THE BUREAU OF FISHERIES. 



curve shows that two of the regions where chemical action is going on most 

 vigorously are the thermocline and the bottom water. The accumulation and 

 decomposition of the plankton in the lower water is a sufficient explanation 

 for the changes which take place there. The reduction at the thermocline is 

 apparently due to the fact that the algae, as they begin to die and sink, often 

 remain for some time at the thermocline. The cool water apparently causes 

 their life to be prolonged, and while certain parts of the filaments are dead 

 and decomposing, others still retain sufficient vitality to keep the plant from 



sinking. When this period is 

 C T ■" 



A 

 I 



,4 2 ? 4 .6 6. 10 lg 14. 16. 18 



30 12 2A 36 38 40 



2 



3 

 4 



5 



6 



7 



6 



9 



10 



II 



12 



13 



14 



15 



16 



17 



18 



19 



20 



21 



22 



X. 



passed, the plant sinks steadily 

 and rather rapidly to the bot- 

 tom, thus consuming compara- 

 tively little oxygen on the jour- 

 ney. Many forms of plankton 

 animals also accumulate in the 

 thermocline, finding there more 

 food than in any other part 

 of the cool water. Both these 

 causes, then, lead to a diminu- 

 tion of the oxygen at this point. 

 It must not be supposed that 

 there has been no loss of oxygen 

 in the lower water where it is at 

 a maximum. In early spring 

 this water would have contained 

 between 8 cubic centimeters and 

 9 cubic centimeters per liter, so 

 that at least one-third of the 

 original stock has been con- 

 sumed. 



Green Lake is the only lake 

 in Wisconsin that shows so small 

 a reduction of the oxygen of the 

 lower water. Lake Geneva, near the southern boundary of the state, has about 

 the same dimensions as Green Lake; it is the second deepest lake in the State, 

 having a depth of 41 meters. The late summer conditions are shown in figure 

 13, where the same general facts are visible as in Green Lake; but the oxygen is 

 much more reduced and shows only a trace, or is altogether absent, from the 

 depth of 33 meters to the bottom. In North Lake, one of the Oconomowoc 

 group, consumption of oxygen at the thermocline goes on more rapidly, and 

 sometimes leads, as is shown in figure 14, to the disappearance of the oxygen 

 from that stratum, while some of the gas still remains at a greater depth. 



C T 



Fig. 10. — Lake Mendota, October 8, 1906. 



Cb 



